The invention concerns active antennas constituted by a large number of array elements excited by microwave transmit power amplifiers and the received signals from which are amplified by low-noise receive amplifiers. These antennas are used in diverse applications including telecommunications and radar; the invention is particularly advantageous in the case of radar. In the field of radar, the usual monostatic radar architecture uses a transmit channel and a receive channel connected to the same array element. A switch is usually employed to select the transmit channel to send a pulsed radar signal, the interval between the pulses transmitted being used to receive radar echoes returned from the environment after selecting the receive channel.